1. Field of the invention.
The present invention relates to a method of an optical measuring of a position including the provision of a photosensitive element having a plurality of photodiodes equidistantly spaced apart from each other and forming a line grid, and of an optical scale which is displaceable relative to the photosensitive element and which scale is projected onto the photosensitive element, the projected line density of the scale being different from the line density of the photosensitive element and the phase relationship of the interference pattern generated thereby being detected in order to determine the mutual relative position of the two line grids.
2. Description of the prior art.
Various methods of an optical measuring of a position are known by means of which a high resolution can be arrived at. The lengths of measurement which can be arrived at by such methods are, however, limited. Due to this fact optical position measuring methods of the kind mentioned above have been applied mainly in angle measuring apparatuses, such as for instance in modern theodolites.
The German published patent application DE-OS No. 3150349 discloses for instance an angle detector of high resolution for use in a theodolite. It encompasses basically the principle of the high resolution by mans of line grids and an array of photodiodes, i.e. the principles of a detection of positions by means of optical interference of line grids having varying line densities, or a line grid and photodiode array, respectively.
The outputs of photodiode arrays of this known apparatus are subjected to an analogue processing, for instance by using CCD's which are generally available on the market.
In case an absolute value measurement is to be carried out besides the high resolution two separate diode arrays are proposed which are arranged such that the projected line grid in the second array is displaced in phase relative to the first array by 90.degree. such as to thereby incrementally detect the absolute angular rotation.
A further possibility proposed for detecting the absolute value is a separate optical trace having a binary Gray-code.
The European patent application No. 83 109 286.1 describes also explicitly the possibility of a high resolution by means of a line grid and of a photodiode array for a measuring and detecting of a linear or curvilinear movement of a scale. However, the specification lacks an exact explanation as to how the large number of photodiode connections are to be lead to a signal processor and processed thereat. It is mentioned, furthermore, in very general terms that the principle may be realized not only on an optical, but also on an electrostatic (capacitive) or magnetic basis.
The disclosed method and apparatus are obviously suitable only for a detecting of very small or tiny, movements.
The European applications No. 84 100 465.8 and No. 84 100 697.6 describe also measuring apparatuses which are based on the already mentioned principle. In this case separate reference markings are located aside of the regular line grid trace for the incremental high resolution such as to enable a detecting of absolute values.